Small particles of nickel sulfide (NiS) usually known as nickel sulfide "stones" sometimes occur in glass and result in severe degradation of the glass quality. Nickel sulfide stones are usually too small to be seen and are very difficult to be detected by optical inspection means. Their negative effect on glass products is a result of the large difference between the coefficient of thermal expansion of nickel sulfide and that of glass. A change in temperature of a glass product, such as a glass sheet installed in a building or vehicle, that includes a nickel sulfide stone can cause intense, localized stresses to be created in the vicinity of the stone which can be of sufficient magnitude to fracture the sheet. This problem is particularly acute in tempered glass. Nickel sulfide stones also undergo slow phase changes which create local stresses. Because it is difficult to detect the presence of nickel sulfide stones in glass, and because their effects may not be exhibited until long after the glass sheet has been installed, prevention of nickel sulfide stones is an important objective for glassmakers.
The most straightforward approach to avoiding nickel sulfide stones is to prevent any source of nickel from entering the glass melting furnace. But trace amounts of nickel can appear as naturally occurring impurities in the raw materials used for making glass. Also, the common presence of nickel in stainless steel alloys used in equipment associated with the raw material mining and handling and in other machinery associated with a glass melting operation can lead to the inadvertent introduction of small amounts of nickel into a glass melting furnace even when strenuous efforts are made to avoid its deliberate introduction.
It would be desirable if formation of nickel sulfide stones could be prevented in the output from a glass melting furnace in which trace amounts of nickel may be present, and it is to this objective that the present invention is directed.
The prior art that may be considered to most closely resemble the present invention are those that deal with electrolysis of glass, but none of these teaches the prevention of nickel sulfide stones and none discloses an arrangement that would inherently produce that result. Passage of electrical current through molten glass in a melting furnace is a common practice for the purpose of assisting the heating of the glass. Conventionally, alternating current is used for this purpose, and therefore no electrolysis of the glass is involved. A few proposals have been disclosed for using direct current electrolysis for specialized purposes.
In U.S. Pat. Nos. 3,811,858 (Ernsberger et al.) and 3,811,859 (Ernsberger) electrolysis is used to create oxygen bubbles to enhance the rising convection current known as the "spring zone." To yield that effect the anodes (26) are located directly under or in the spring zone and the cathodes (28) are located on the floor of the furnace upstream from the anode in both of these patents. These locations are inappropriate for preventing nickel sulfide stones in accordance with the discoveries of the present invention.
A similar arrangement is shown in U.S. Pat. No. 4,433,995 (Toussaint). The anodes (17, 18) are located in the spring zone or downstream therefrom while the cathodes (13, 14, 15, 16) are located near the batch inlet for the sake of assisting the melting of the batch. These electrode locations would not achieve the results of the present invention.
U.S. Pat. No. 1,955,451 (Blau) uses electrolysis to separate a glass melt into two compositionally different fractions after melting is completed.
U.S. Pat. No. 3,530,221 (Penberthy) discloses the imposition of a direct current component onto an alternating current electric melting circuit for the sake of preventing corrosion of an electrode.
In U.S. Pat. No. 2,561,818 (Peyches) the walls of a glass melting vessel are disclosed to be preserved by applying a charge to the wall itself.
U.S. Pat. No. 4,227,029 (Joseph), based on the conventional belief that direct current is undesirable in a glass melting operation, discloses an arrangement for minimizing direct current components in an alternating current melting circuit.
U.S. Pat. Nos. 2,277,678 and 2,281,408 (both Borel) show positive and negative signs on electrodes in the drawings, but otherwise describe conventional electric melting principles consistent with the use of alternating current. No electrolysis is mentioned, and the relationship of the electrodes to the glass currents is not disclosed.